116 INFERIOR OOLITE AMMONITES. 



Inferior- Oolite species) is a long interval, during which no species belonging to 

 this genetic series has been recorded. In fact the occurrence of a species in the 

 Sauzei-zone is a new find, and helps to decrease what was, and is even now, a 

 very long break in the record. Ammonites Mercati differs from these species, 

 because its ribs are not really sigmoidal, but are straighter on the lateral area, and 

 more conspicuous on the inner area, and its inferior lateral lobe is a trifle smaller. 

 Am. Mercati could be included in the genus Pcecilomorplms under the same 

 conditions as Hild. Kiliani and H. Douvillei are in Hildoceras. The development of 

 the ribs of the one genus is paralleled by that of the other, — the older species 

 possess straight ribs, the younger have sigmoidal ribs ; in other words, in these 

 two genera the older species retain the Arietan form of ribbing, the younger possess 

 the Harpoceras mode. 



Poecil. Mercati shows the same involute and evolute varieties as Poecil. cycloides ; 

 but apparently it does not vary in thickness in a similar manner. The shape of 

 Poecil. macer is very different from Poecil. cycloides ; it is in fact more like that of 

 Hildoceras. On the other hand, Poecil. Mercati has considerable similarity to 

 certain species of Lillia, while Poecil. cycloides, by its peculiar shape and orna- 

 mentation, is easily separable from both Lillia and Hildoceras ; yet we can see how 

 these species of Poecilomorplms are bound up together as the members of one genus 

 or branch. This branch, I take it, is more closely allied to Hildoceras than it is to 

 Lillia ; that is to say, Lillia was the first by some time to leave the parent stem, 

 and to assume its own distinctive characters. 



In the Humphriesianum-zone is found the chief development of Poecilomorplms, 

 namely, in the numerous varieties of Poecil. cycloides. Poecil. Mercati, which is 

 included in the genus somewhat on sufferance, is a very much earlier species, and 

 is outside the scope of this Monograph. 



Pcecilomorphus macer, 8. Buckman. Plate XXII, figs. 23 — 29 ; Plate A, fig. 33. 



Discoidal, compressed, very evolute, carinate; whorls, subquadrate with 

 slightly convex sides, ornamented with sigmoidal radii, of which the outer arc 

 alone is conspicuous and is longer than the inner. The radii, which become very 

 faint again on the ventral area, are projected forwards to join the carina at an 

 acute angle. The ventral area carries a small solid carina, on each side of which 

 runs a small furrow. Inner margin convex. Inclusion about one third. 

 Umbilicus flat and wide. Termination plain, with a projected, rounded lateral, 

 and a pointed ventral, process. Body-chamber at least O'GO of a whorl in length. 



